Children at play often move away or out of sight of parents or other guardians having responsibility for them. At such times, it would be desirable for a guardian to know if a child has wandered more than a predetermined distance away as this may indicate that the child is lost or in trouble. It would also be desirable quickly to locate or trace that child.
Furthermore, when mobile phones are lost or stolen, it would be useful for the owner to know if the phone has been removed more than a predetermined distance from him. It would be desirable to be able to disable a lost or stolen mobile phone.